Mobile station

ABSTRACT

A mobile station is provided for autonomously detect an abrupt change of a propagation environment of a radio wave signal from a base station for servicing a peripheral cell and immediately executing a peripheral cell search. In order to execute a received signal strength indication plural times within one period of the peripheral cell search, the mobile station operates to set a cell search execution period Δt1 to a timer of a peripheral cell search controller and a RSSI measuring execution period Δt2 to a timer. The RSSI measuring unit operates to measure the RSSI in the receiving band at each RSSI measuring execution period Δt2 and immediately execute the peripheral cell search when the peripheral cell search controller determines that the RSSI has greatly varied.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile station included in a mobilecommunication system such as a mobile telephone or a portable telephone,and more particularly to a mobile station which is arranged toautonomously detect an abrupt change of the quality of a signal receivedfrom a peripheral cell and then immediately start to search peripheralcells.

2. Description of the Related Art

A third-generation mobile communication system is planning to employ aCDMA (Code Division Multiple Access) system as a radio access system.The CDMA system is arranged to execute a soft hand-over operation forsynthesizing a radio signal from a base station of a current cell (wherethe mobile station is located) with a radio signal from a base stationof a new cell (where the mobile station is moved) as a hand-over whenthe mobile station is moved from the current cell to the next cell. Inorder to execute the hand-over, the mobile station is required to do aperipheral cell search for searching for a base station of the nextcell.

Each base station is constantly transmitting a radio channel called aperch (or pilot) channel. The perch channel is transmitted through theuse of a diffusion (or spreading) code which is peculiar to that cell(base station). Hence, each base station notifies the mobile station ofthe diffusion code used in that cell on the perch channel. The mobilestation operates to periodically measure the ratio of a target signal tointerference noise (SIR: signal-to-interference ratio) of the radiochannel (the perch channel of the peripheral cell) that corresponds tothe notified diffusion code and then notifies the network of themeasured result. Herein, the SIR has the same meaning as the “receivedsignal quality”, which will be often used below. This operation iscalled a peripheral cell search. The network determines to which cellthe mobile stations is moving.

The action of the conventional mobile station about executing theperipheral cell search will be described with reference to FIG. 1

(Step 1)

The mobile station (terminal) receives the perch channel, obtains theinformation about diffusion codes used by the peripheral cells, andstores it.

(Step 2)

The mobile station sets a cell search execution period Δt to a timer.When the timer is counted up, the operation goes to a step 3.

(Step 3)

A reverse diffusion (or de-spreading) process is executed bysequentially using the stored diffusion codes for the peripheral cells,for the purpose of measuring the SIR of each cell.

(Step 4)

The mobile station notifies the network of the result of the peripheralcell search. Herein, the description is explained in the case where theresult is reported to the network each time the peripheral cell searchis executed. However, the report is not necessarily required for eachperipheral cell search. After the result of the peripheral cell searchis reported to the network, the operation goes back to the Step 2.

As described above, the mobile station executes the peripheral cellsearch at a period Δt and reports the received signal quality from theperipheral cell to the network. If the network determines the mobilestation is to be shifted to the peripheral cell based on the reportabout the received signal quality, the network gives an instruction of asoft hand-over with a target base station (where the mobile station isto be moved) to the mobile station.

However, the foregoing peripheral cell search is executed to measure thereceived signal quality from the peripheral cell at each period Δt.Hence, if the received signal quality from the peripheral cell isabruptly changed, disadvantageously, in the worst case, the abruptchange cannot be detected until Δt is passed. The abrupt change of thereceived signal quality from the peripheral cell may take place when abase station is moved from a place located out of sight, concretely,behind a building to a place located in sight with movement of themobile station or when continuous changes take place from one basestation located in sight to another one in the case of a mobile stationmoving in a mountain area.

In order to detect the change of the received signal quality from theperipheral cell as soon as possible, a method may be taken for reducingΔt. This method, however, requires the mobile station in communicationto execute the peripheral cell search with the reverse diffusion at ashort period. This peripheral cell search thus increases the processingamount of the mobile station and thereby increases the electric currentconsumption which results in reducing the using time of the mobilestation.

On the other hand, a method for dynamically tracing a mobile station ina cellular network is disclosed in U.S. Pat. No. 5,390,234. In thismethod, only after the mobile station is turned on and moved by aminimum distance, does the mobile station gives a report to the basestation. If the base station needs to communicate with the mobilestation, the mobile station is searched only close to the last cellreported by the mobile station to the base station. However, this methodprovides no capability of coping with an abrupt change of the receivedsignal quality from the peripheral cell.

Further, JP-A-3-268697 discloses a mobile radio communication system inwhich the mobile station itself constantly monitors the radio linequality based on the received field strength and a code error rate of ademodulated signal while the mobile station is moving over a radio zone.When the mobile station detects a degradation of the received signalfield strength or the code error rate owing to the movement over theradio zone, the mobile station sequentially compares one base stationhaving a frequency for the current zone through an empty time slot withanother base station in light of the field strength of the signal, forthe purpose of determining the signal having the strongest field. Themobile station transmits the frequency information of the determinedsignal to the base station that is communicating with the subject mobilestation, for requesting a hand-over. Then, the base station transmits amessage for requesting the hand-over to a control station. In responseto the message, the control station performs the hand-over. However,this system enables to switch the channel only when the average receivedsignal level is abruptly lowered. Conversely, if the average receivedsignal level is abruptly enhanced, it means that the mobile station islocated in the area where lots of obstacles are located. In this area,the mobile station may utilize a more favorable base station.Regrettably, for coping with such a case, the system does not considerselection of a more favorable base station.

Further, JP-A-5-91038 discloses a system for switching channels ofmobile communication, in which system, when the variable width of thereceived signal level stays in a predetermined range, the channelswitching level is set to a higher value and when the average receivedsignal level is abruptly lowered, the channel is quickly switched. Thissystem, however, does not have any suggestion of selecting a morefavorable base station when the average received signal level isabruptly increased.

Moreover, JP-A-7-203517 discloses a hand-over system having the functionof identifying whether a radio base station in communication and a radiobase station adjacent thereto are an indoor radio base station or anoutdoor radio base station and the function of measuring the fieldstrength received from the adjacent radio base station and a receivedfield strength variation per unit time. In this system, when the mobilestation is communicating with the indoor radio base station and thereceived field strength and the received field strength variation perunit time from the adjacent outdoor radio base station become more thantheir predetermined values, the mobile station requires a channel changeto the outdoor radio base station irrespective of the received fieldstrength from the indoor radio base station in communication. Thissystem, however, may select the base station that does not have the bestreceived field strength or SIR.

As another prior art, JP-A-8-223107 discloses a channel switchingdetermining device for a mobile radio terminal, in which device requestfor switching channels are suppressed to a minimum while the mobileradio terminal is moving around an office area with lots of buildings,for the purpose of keeping the communication stable. This device isarranged so that the mobile radio terminal requires to switchcommunication channels when an average value of bit error rates isinferior to a predetermined value and an average value of received fieldstrengths is lower than a predetermined value. However, this device isnot able to cope with an abrupt change of the received signal qualityfor a period which is shorter than the period when the average value ofthe bit error rates is measured.

SUMMARY OF THE INVENTION

The present invention is made for overcoming the foregoing shortcomings.It is an object of the present invention to provide a mobile stationwhich includes means for measuring a received signal strength indicatoror indication (carrier RSSI, or simply RSSI) of a signal inside of areceiving area before the reverse diffusion with a simple process andwithout having to do a reverse diffusion process and operates torecognize an abrupt change of a received signal quality from anyperipheral cell by detecting a change of the RSSI of the signal insideof the receiving area.

In carrying out the object, according to an aspect of the invention, amobile station for a CDMA system mobile communication system includestransmitting and receiving means for transmitting and receiving a signalto and from one of plural base stations servicing peripheral cells,received signal level measuring means for measuring a received signalstrength indication (RSSI) within a receive band, in the signal receivedby the transmitting and receiving means, reverse diffusing means forperforming a reverse diffusing process with respect to the signalreceived by the transmitting and receiving means, received signalquality measuring means for measuring a ratio of a target signal tointerference noise (SIR) at an output of the reverse-diffusing means,and control means for searching the peripheral cells and controlling themeasuring means to measure the SIR at certain periods and at least whenchange of the RSSI output from the received signal level measuring meansexceeds a predetermined value, and for reporting the received signalquality of the peripheral cells to the network through the base stationin communication so as to shift the mobile station to a soft hand-overstate.

The foregoing arrangement makes it possible to implement an excellentmobile station which operates to autonomously detect an abrupt change ofthe received signal quality from the peripheral cells and immediatelyperform a peripheral cell search by measuring the received signal levelfor a shorter period than the period at which the peripheral cells aresearched.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart for illustrating the operation of searching aperipheral cell in a conventional mobile station;

FIG. 2 is a block diagram showing an arrangement of a mobile stationaccording to a first embodiment of the invention;

FIG. 3 is a flowchart for illustrating the operation of searching aperipheral cell in the mobile station according to the first embodimentof the invention;

FIG. 4 is a block diagram showing an arrangement of a mobile stationaccording to a second embodiment of the invention;

FIG. 5 is a flowchart for illustrating the operation of searching aperipheral cell in a mobile station according to the second embodimentof the invention; and

FIG. 6 is a block diagram showing an arrangement of a mobilecommunication system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the embodiments of the invention will be described withreference to FIGS. 2 to 6.

(First Embodiment)

FIG. 2 is a block diagram showing an arrangement of a mobile station forexecuting a peripheral cell search according to a first embodiment ofthe invention. In FIG. 2, the mobile station includes a radio signaltransceiver 11 for transmitting and receiving a radio signal to and froma base station, a reverse diffuser (or de-spreading) 12 for reverselydiffusing (or de-spreading) a diffused signal received by the radiosignal transceiver 11 by using diffusion (or spreading) codes and takinga signal directed to the subject station, a received signal qualitymeasuring unit 13 for measuring the SIR (signal-to-interference ratio)from a signal sent from the reverse diffuser 12, a diffusion code (orspreading code) storage unit 14 for storing the diffusion codes used inthe peripheral cells and reported on a perch (or pilot) channel, aperipheral cell search controller 15 for executing a peripheral cellsearch at a pre-set timer period, the controller 15 being provided withtimers T1 and T2, a message generator 16 for generating a message forreporting to the network the result of the peripheral cell searchperiodically executed by the peripheral cell search controller 15, and areceived signal level measuring unit 17 for measuring a received signalstrength indicator or indication (carrier RSSI, or simply RSSI) of areceiving band signal sent from the radio signal transceiver 11.

Now, the description will be oriented to the operation about executing aperipheral cell search in the mobile station arranged as described aboveaccording to the first embodiment of the invention with reference toFIG. 3.

(Step 1)

The mobile station (terminal) receives a signal on the perch channel,obtains the diffusion code information used by the peripheral cells, andstores them in the diffusion code storage unit 14.

(Step 2)

peripheral cell search controller 15 operates to set a cell searchexecution period Δt1 to the timer T1.

(Step 3)

The peripheral cell search controller 15 operates to set a receivedsignal level measurement execution period Δt2 to the timer T2.

Here, the relation Δt2<Δt1 is required. Setting Δt2=Δt1/k (k=2, 3, 4, .. . ), by executing the detection of the received signal level signal ktimes within one period of the peripheral cell search, the detection canbe executed at 1/k time of the peripheral cell search period. Hence, themobile station can recognize an abrupt change of the received signalquality from the peripheral cell.

(Step 4)

When the timer T2 is counted up, the peripheral cell search controller15 instructs the received signal level measuring unit 17 to measure thereceived signal strength indication (RSSI).

(Step 5)

The peripheral cell search controller 15 determines the magnitude of thechange of the RSSI of the receiving band signal, based on the levelmeasured result from the RSSI measuring unit 17. For example, thiscontroller 15 may set a threshold value A for determining the presenceof an environmental change of radio propagation from the peripheralcell, if the following relation is met for the n-th RSSI detected resultR(n):

|R(n−1)−R(n)|≧A

If the change of the RSSI is determined to be large, the operation goesto a step 6. That is, it is presumed that there exists a base station tobe detected by the peripheral cell search.

If the change of the RSSI is determined to be small,

(1) if the timer T1 is counted up, the operation goes to step 6.

(2) if the timer T1 is not counted up, the operation goes to step 3.

(Step 6)

The mobile station operates to sequentially set to the reverse diffuser12 the diffusion codes used in the peripheral cells, which are stored inthe diffuse code storage unit 14. Then, the received signal qualitymeasuring unit 13 operates to measure the SIR from each cell.

Afterwards, the mobile station operates to report the result of theperipheral cell search to the network. Herein, the report to the networkat each peripheral cell search has been described as an example. Inactuality, a report at each search is not necessarily required.

After reporting the result of the peripheral cell search to the network,the operation goes to the Step 2.

The mobile station executes the peripheral cell search along theprocedure from the Step 1 to the Step 6.

As described above, according to the first embodiment of the invention,if the propagation environment of a radio signal from a base station forservicing the peripheral cells is abruptly changed, the mobile stationquickly detects the change within a time period Δt1 in the worst case,executes the peripheral cell search, and shifts to the soft hand-overstate with a proper base station.

(Second Embodiment)

The second embodiment of the present invention is concerned with amodification of the mobile station according to the first embodiment,which determines whether or not the environment around the subjectstation has lots of obstacles (buildings or mountains) for obstructingthe wave propagation by measuring the period of RSSI variations in thereceiving band measured by the RSSI measuring unit, and changes theperiod of the peripheral cell search to a shorter period if theenvironment is determined to have lots of obstacles or to a longerperiod if it is determined not to have lots of obstacles. As such, themobile station provides the capability of autonomously determiningwhether or not the environment around the mobile station has lots ofobstacles and adaptively increasing or decreasing the period at whichperipheral cell searches are executed.

FIG. 4 is a block diagram showing an arrangement of a mobile station forexecuting the peripheral cell search according to the second embodimentof the invention. In FIG. 4, the mobile station includes a radio signaltransceiver 31 for transmitting and receiving a radio signal to and froma base station, a reverse diffuser 32 for reversely diffusing a diffusedsignal received by the radio signal transceiver 31 by using diffusioncodes and taking a signal oriented for the subject station, a receivedsignal quality measuring unit 33 for measuring the SIR, a diffusion codestorage unit 34 for storing the diffusion codes used in the peripheralcells and reported on a perch channel, a peripheral cell searchcontroller 35 for executing a peripheral cell search at a pre-set timerperiod, the controller 35 being provided with timers T1 and T2, amessage generator 36 for generating a message for notifying the networkof the result of the peripheral cell search periodically executed by thecontroller 35, and a RSSI measuring unit 37 for measuring the RSSI ofthe receiving band signal sent from the radio signal transceiver 31.

The description will be oriented to the operation about the peripheralcell search executed by the mobile station arranged as described aboveaccording to the second embodiment of the invention with reference toFIG. 5.

(Step 1)

The mobile station receives a signal on the perch channel, obtains theinformation about diffusion codes to be used by the peripheral cells,and stores it in the diffusion code storage unit 34.

(Step 2)

The peripheral cell search controller 35 operates to set a cell searchexecution period Δt1 to the timer T1.

(Step 3)

The peripheral cell search controller 35 operates to set a RSSImeasuring period Δt2 to the timer T2.

Herein, the relation Δt2<Δt1 is required. If Δt2=Δt1/k (k=2, 3, 4, . . .), by executing the detection of the received signal level k timeswithin one period of the peripheral cell search, the detection can beexecuted at 1/k time of the peripheral cell search period. Hence, anmobile station can recognize the abrupt change of the received signalquality from the peripheral cell.

(Step 4)

When the timer T2 is counted up, the peripheral cell search controller15 instructs the received signal level measuring unit 17 to measure thereceived signal strength indication (RSSI).

The level variation period measuring unit 38 stores the RSSI measuredresult as the RSSI varying history.

(Step 5)

The peripheral cell search controller 35 determines the magnitude of theRSSI change in the receiving band signal, based on the RSSI measuredresult sent from the RSSI measuring unit 37 (Peripheral EnvironmentDetermination 1).

For example, this controller 35 may set a threshold value A fordetermining the presence of an environmental change of radio propagationfrom the peripheral cell, if the following relation is met for the n-thRSSI detected result R(n):

|R(n−1)−R(n)|≧A

If the change of the RSSI is determined to be large, the operation goesto a step 6. That is, it is presumed that there exists a base station tobe detected by the peripheral cell search.

If the change of the RSSI is determined to be small,

(1) if the timer T1 is counted up, the operation goes to step 6.

(2) if the timer T1 is not counted up, the operation goes to step 3.

(Step 6)

The mobile station operates to sequentially set to the reverse diffuser32 the diffusion codes used in the peripheral cells, which are stored inthe diffuse code storage unit 34. Then, the received signal qualitymeasuring unit 33 operates to measure the SIR from each cell.

Afterwards, the mobile station operates to report the result of theperipheral cell search to the network. Herein, the report to the networkat each peripheral cell search has been described as an example. Inactuality, the report at each search is not necessarily required.

After reporting the result of the peripheral cell search to the network,the operation goes to the Step 2.

(Step 7)

The level variation period measuring unit 38 operates to measure theRSSI variation period based on the RSSI variation history. For example,the measuring unit 38 may operate such that the RSSI measured resultfrom the RSSI measuring unit 37 is quantized at L stages and the timewhen the quantized value is not varied is measured as the RSSI variationhistory.

The peripheral cell search controller 35 operates to determine whetheror not the peripheral environment has lots of obstacles, based on theRSSI variation period measured by the RSSI variation period measuringunit 38. For example, the controller 35 may operate such that athreshold value B for determining whether or not the environment haslots of obstacles is pre-set to the measured result LT of the RSSIvariation period; and

(1) If LT<B, the environment is determined to have lots of obstacles andΔt1 is decreased accordingly, or

(2) If LT≧B, the environment is determined not to have lots of obstaclesand Δt1 is increased accordingly.

Along the procedure of these Steps 1 to 7, the mobile station executesthe peripheral cell search.

As described above, according to the second embodiment of the presentinvention, the mobile station operates to autonomously determine whetheror not the environment around the mobile station itself has lots ofobstacles and set a proper period for the peripheral cell search basedon the determined result. Hence, if the environment therearound has lotsof obstacles, the mobile station executes the peripheral cell search ata short period for corresponding to the change of the received signalquality from the peripheral cell. If the environment therearound doesnot have lots of obstacles, the mobile station executes the peripheralcell search at a long period. This procedure makes it possible for themobile station to eliminate unnecessary peripheral cell searches.

Moreover, as shown in FIG. 6, a mobile communication system may bearranged to have the mobile station described according to the foregoingembodiments and base stations 1 to N. This mobile communication systemprovides a capability of suppressing an increase of interference if themobile station is moved from an out-of-sight place to an in-sight place.This arrangement makes it possible to implement a mobile communicationsystem for suppressing an increase of interference in the case of movingthe mobile station from the out-of-sight place to the in-sight place.

As set forth above, according to the present invention, if thepropagation environment of the radio signal from the base station forservicing the peripheral cell is abruptly changed, the mobile stationoperates to autonomously detect the change, immediately execute theperipheral cell search, and report the quality measured result to thenetwork. The network can instruct the soft hand-over to the mobilestation.

What is claimed is:
 1. A mobile station in a mobile communication systemof a CDMA (Code Division Multiple Access) type, said mobile stationcomprising: transceiver means for transmitting and receiving a wide-banddiffused signal to and from each of a plurality of base stationsservicing respective ones of peripheral cells; reverse diffusing meansfor reversely diffusing or despreading the wide-band diffused signalreceived by said transceiver means; received signal level measuringmeans for measuring a received signal strength indicator (carrier RSSI)specifying wide-band received power measured relative to the wide-bandsignal received by said transceiver means before the reverse diffusingor despreading by said reverse diffusing means; received signal qualitymeasuring means for measuring a signal to interference noise ratio (SIR)at an output of said reverse diffusing means; and control means forexecuting a search among said peripheral cells and controlling saidreceived signal quality measuring means to measure the SIR at certainperiods and at least when change of the RSSI output from said receivedsignal level measuring means exceeds a predetermined value, and forreporting received signal quality from said peripheral cells to anetwork through a base station in communication with said mobile stationso as to shift said mobile station to a soft hand-over state.
 2. Amobile station according to in claim 1, further comprising: levelvariation period measuring means for measuring a variation period of theRSSI output from said received signal level measuring means anddetermining whether or not the environment around said mobile stationhas lots of obstacles which obstruct propagation of a radio wave;wherein said control means operates to change a searching period of saidperipheral cells to a shorter period when said environment is determinedto have lots of obstacles and to change the searching period of saidperipheral cell to a longer period when said environment is determinednot to have lots of obstacles.
 3. The mobile communication systemincluding the mobile station according to claim 1 and a plurality ofbase stations, wherein when said mobile station is moved from anout-of-sight place of one of said base stations to an in-sight placethereof, an increase of interference is suppressed.
 4. The mobilecommunication system including the mobile station according to claim 2and a plurality of base stations, wherein when said mobile station ismoved from an out-of-sight place of one of said base stations to anin-sight place thereof, an increase of interference is suppressed.
 5. Animproved mobile station for use in a Code Division Multiple Accesscellular communication system which includes a plurality of basestations that are arranged in a network and that serve respective cells,the mobile station including a transceiver and means responsive to asignal received by the transceiver for conducting peripheral cellsearches to measure the quality of signals transmitted by the basestations of cells peripheral to a current cell, at least some of thesignal quality measurements being transmitted via the transceiver to oneof the base stations for transfer to the network and use whenhanding-over the mobile station from the current cell to one of theperipheral cells, the means including a first timer for timing a normalinterval between peripheral cell searches, wherein the improvementcomprises: the means responsive to a signal received by the transceiverfor conducting peripheral cell searches additionally includes a receivedsignal strength measuring unit to measure the strength of the signalreceived by the transceiver, a second timer for timing shortenedintervals that are shorter than the normal interval, and means forconducting a peripheral cell search before the normal interval expiresif the strength of the signal received by the transceiver varies by atleast a predetermined amount during one of the shortened intervals. 6.The mobile station of claim 5, wherein the shortened interval is 1/ktimes the normal interval, k being an integer greater than one.
 7. Themobile station of claim 6, wherein the means responsive to a signalreceived by the transceiver for conducting peripheral cell searchesadditionally includes means for increasing the normal interval if thestrength of the signal received by the transceiver is changing slowly.8. The mobile station of claim 7, wherein the means responsive to asignal received by the transceiver for conducting peripheral cellsearches additionally includes means for reducing the normal interval ifthe strength of the signal received by the transceiver is changingrapidly.
 9. The mobile station of claim 5, wherein the means responsiveto a signal received by the transceiver for conducting peripheral cellsearches additionally includes means for increasing the normal intervalif the strength of the signal received by the transceiver is changingrapidly and for reducing the normal interval if the strength of thesignal received by the transceiver is changing slowly.
 10. An improvedmobile station for use in a Code Division Multiple Access cellularcommunication system which includes a plurality of base stations thatare arranged in a network and that serve respective cells, the mobilestation including a transceiver and means responsive to a wide-banddiffused signal received by the transceiver for conducting peripheralcell searches to measure the quality of signals transmitted by the basestations of cells peripheral to a current cell, at least some of thesignal quality measurements being transmitted via the transceiver to oneof the base stations for transfer to the network and use whenhanding-over the mobile station from the current cell to one of theperipheral cells, the means including a reverse diffuser for reverselydiffusing or despreading the wide-band diffused signal received by thetransceiver, and a first timer for timing a normal interval betweenperipheral cell searches, wherein the improvement comprises: the meansresponsive to a signal received by the transceiver for conductingperipheral cell searches additionally includes a received signalstrength measuring unit to measure the strength of the wide-banddiffused signal received by the transceiver before it is reverselydiffused or despread by the reverse diffuser, a second timer for timingshortened intervals that are shorter than the normal interval, and meansfor conducting a peripheral cell search before the normal intervalexpires if the strength of the wide-band diffused signal received by thetransceiver varies by at least a predetermined amount during one of theshortened intervals.
 11. The mobile station of claim 10, wherein theshortened interval is 1/k times the normal interval, k being an integergreater than one.
 12. The mobile station of claim 11, wherein the meansresponsive to a signal received by the transceiver for conductingperipheral cell searches additionally includes means for increasing thenormal interval if the strength of the signal received by thetransceiver is changing slowly.
 13. The mobile station of claim 12,wherein the means responsive to a signal received by the transceiver forconducting peripheral cell searches additionally includes means forreducing the normal interval if the strength of the signal received bythe transceiver is changing rapidly.
 14. The mobile station of claim 10,wherein the means responsive to a signal received by the transceiver forconducting peripheral cell searches additionally includes means forincreasing the normal interval if the strength of the signal received bythe transceiver is changing rapidly and for reducing the normal intervalif the strength of the signal received by the transceiver is changingslowly.